We have studied lymphocyte aryl hydrocarbon hydroxylase (AHH) activity and its inducibility in normal humans and leukemia patients. We have also investigated the effect of various factors that influence lymphocyte AHH activity during culture. Our data suggest that human lymphocyte AHH activity appears in lymphocytes cultured in the presence of various mitogens: phytohemagglutinin (PHA), Concanavallin A (Con. A), Poke weed mitogen (PWM) and various combinations of these mitogens. Response in normal as well as leukemia patients was greatest with T-cell specific mitogens (e.g. PHA, Con. A) than with B-cell specific mitogens, especially PWM. Various factors, such as the type of mitogen, the batch of fetal calf serum, the type of inducer, counting of harvested cells, storage of blood or lymphocytes, type of culture container and the length of incubation during culture, were found to affect both basal and induced AHH activity. Under optimum conditions the method currently used in our laboratory to measure AHH inducibility gives constant values in repeat determinations on the same donor in greater than 80% cases with an average coefficient of variation of 0.13. BIBLIOGRAPHIC REFERENCES: Gurtoo, H.L., Motycka, L.E. and Parker, (Bejba), N., Sex Dependence of the Metabolic Activation In Vitro of the Mycohepatocarcinogen Aflatoxin B1. J. Medicine, 7:1-12, 1976. Gurtoo, H.L., Sex Differences in the Metabolism and Metabolic Activation of Aflatoxin B1 (AFB1) by Rat Hepatic Microsomes, Federation Proceedings, 1976, 35(3):330, 1976.